Electrostatic image reproduction



Jan. 2, 1962 c. F. CARLSON ET AL 3,015,304

ELECTROSTATIC IMAGE REPRODUCTION Filed Oct. 2, 1957 2 Sheets-Sheet 1 56 HIGH VOLTAGE SOURCE A.C. SOURCE HIGH- VOLTAGE SOURCE FIG. 1

INVENTORS Chester F. Carlson By Harold Bogdonoff I ww AT'LORNEY Jan. 2, 1962 c. F. CARLSON ET AL 3,015,304

ELECTROSTATIC IMAGE REPRODUCTION 2 Sheets-Sheet 2 Filed Oct. 2, 1957 INVENTORLY. Chester F. Carlson B Harold Bogdonoff ATTORNEY 3,015,364 ELECTRUSTATIQ HVlAiGE REPRUDUCTEQN Chester F. (larlson, Pittsford, and Harold Bogdonolf,

Rochester, N.Y., assignors, by mesnc assignments, to

Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Oct. 2, 1957, Ser. No. 687,669 17 Claims. ((31. 118--637) This invention relates to apparatus for transferring electrostatic images and the application of such apparatus to xerography and other electrostatic image processes. This is a continuation-impart of U. S. patent application Serial No. 591,775, filed June 14, 1956, and now U.S. Patent No. 2,982,647, which is a continuation-in-part of now abandoned US. patent application Serial No. 358,446, filed May 29, 1953.

In particular, the invention contemplates apparatus for the transfer of an electrostatic latent image which has been formed previously on an insulating layer to a second layer of insulating material by bringing the image bearing surface into contact with or into close proximity to a surface of the second layer, applying an intense electric field between these surfaces, and then separating the layers to form a transferred pattern of electric charges on the second insulating layer.

Generally, in electrostatic image processes such as Xerography it has been the practice to develop the electrostatic images on the surfaces on which they are formed by directly depositing solid or liquid material thereon. Alternatively, images have been developed on sheets laid on such surfaces as, for example, is described in connection with FIGURE 9 of Carlson Patent 2,297,691. It has not heretofore been thought possible or practicable to transfer the undeveloped and invisible electrostatic image from one surface to another and, yet, such a transfer would allow for further and advantageous uses of the transferred image.

In our aforementioned patent application, of which this is a continuation-in-part, there are described various methods of transferring charges. This invention is concerned with apparatus to manipulate charges according to the method of the aforementioned applications. Further, this invention is concerned with developing means particularly valuable when used in connection with images formed according to this invention.

In the drawings:

FIGURE 1 illustrates an embodiment of apparatus according to this invention; and,

FEGURE 2 illustrates an embodiment of improved developing apparatus valuable in connection with this invention.

For a better understanding of this invention reference is now had to FIGURE 1 wherein an embodiment of apparatus according to this invention is illustrated. In this embodiment there is illustrated a Xerographic drum generally designated 17 and comprising photoconductive insulating layer 18 which may be any known photoconductive insulating material and, preferably, a uniform layer of vitreous selenium overlying a conductive backing cylinder 20. The drum, as illustrated, is rotated in a clockwise direction and is driven at a uniform speed by motor and gear train 21 connected to drum 17 through belt drive 22. Preferably, the motor of motor and gear train 21 comprises a variable speed motor to allow control of the speed of rotation of drum 17, if desired.

Positioned along the circumference of drum 17 is a corona discharge device generally designated 11 comprising a corona discharge wire 35 partially surrounded by shield 36. A high voltage D.C. source 32 is connected through conductor 31 to discharge wire 35 and shield 36 is maintained at ground potential. As illustrated, backice ing member Zll is also grounded and an electric field exists between ions released by discharge wire 35 and drum 17 causing the deposition of charge on the photoconductive insulator 18 and sensitization of the plate or drum 17. Other techniques for sensitizing the Xerographic drum 17 may be employed and are known to the art. Drum 17 when electrostatically charged is light sensitive and, accordingly, housing 37 is provided to maintain drum 17 in darkness.

Following sensitization or charging, the drum is rotated to exposure station 23 whereat a light pattern is projected from copy drum 25 through lens 26 and through slit 27 to the surface of photoconductive insulating layer 18. Copy drum 25, which carries an original to be reproduced such as a drawing, typewritten matter, or other line, halftone or continuous tone subject matter, is illuminated by lights 28 and is rotated in the direction indicated at a speed synchronized with the speed of movement of drum 17. A separate motor 36 is illustrated in this embodiment connected through belt drive 31 to drive copy drum 25. However, it is to be realized and understood that other means commonly known in the art, for eX- ample, the driving means of drum 17 geared to bring about synchronized rotation of drum 25, may be employed.

Referring to FIGURE 1, following image formation at exposure station 23, drum 17 is rotated to image transfer station 12 whereat transfer web 38 comprising an insulating layer such as a high grade of dry paper, plastic coated paper such as polyethylene coated paper or the like moves into contact with the surface of photoconductive insulating layer 18, an image is formed on web 38 and web 38 is separated from Xerographic drum 17. Web 38 in moving to drum 17 issues from supply roll 40, passes over guide roll 41, then between static removers 4.2 and around contact roll 43 into contact with the surface of photoconductor 18. Positioned above contact roll 43 is precharge station 45 whereat, if desired, an electrostatic precharge may be applied to the surface of web 38 which is to contact the surface of photoconductive insulating layer 18 of drum 17. The web leaves or is separated from drum 17 against separating roller 46 and while against the surface of drum 17 and between contact roller 43 and separating roller 46 a transfer charge from discharge device 47 is applied to the rear surface of transfer web 38. The rear surface of web 338 next passes by neutralizing zone 48 prior to movement of web 38 into developing zone 59. Following development, web 38 passes around guide roll 51, then beneath fuser 52, then between drive rolls 53 to takeup spool 55. While the web moves rum 17 continues to rotate and after movement out of image transfer station 12 it passes beneath discharge lamp 56 and then into light contact with brush 57. Brush 57 may comprise a fur brush or the like and its primary function is to remove lint from the surface of the photoconductive insulator 18 prior to recycling. At this point the drum has completed a cycle of rotation and enters into the zone for sensitizing as a new cycle begins.

Reviewing now in greater detail the happenings as the web moves from point to point, it is seen that the web is supplied from supply roll 40 and next moves to guide roll 41 positioned to guide the web as it moves into the transfer station. Before movement into the transfer station web 38 passes between static removers 42 which in this embodiment comprises two corona discharge electrodes positioned on opposite sides of the web and supplying AC. corona to each surface of web 38. Occasionally as a continuous web of material is drawn from a roll it acquires electrostatic charges on its surface. These charges are irregular in nature and, accordingly, static removers 42 are positioned to provide a controlled flow of charge to the surface of Web 38 to assure an uncharged or neutral web to feed to the other stations of operation Patented Jan. 2, 1962.

in this invention. Static removers 42 are connected to A.C. source 13 and preferably the surrounding shields of the discharge electrodes are Slightly biased to assure neutralization of web d8 moving therebetween as is Well known in the art.

Web 38 is next led over contact roll 43 which preferably is a conductive roller such as an aluminum roller and beneath precharge mechanism 45 which may comprise any of various known electrostatic charging mechanisms and preferably one or more corona discharge wires 15 partially surrounded by shield 16. Discharge wires 15 are connected to high voltage source 32 through switch 56 and shield 16 is maintained at ground potential. Contact roll 4-3 is maintained at ground potential and thus provides a terminal for the electric field to cause a flow of charge or ions from precharge mechanism 45 (the other terminal of the field) to the surface of web 33 when precharge mechanism 45 is energized by closing switch 56. It is pointed out that when precharging corona from precharge mechanism 45 is used to precharge the surface of web 38 as the web moves in contact and around contact roller 43 any irregular charges on web 38 appear to be ironed out, and, accordingly, when precharge mechanism 45 is energized it is generally not necessary to remove static charge with static removers 42 prior to feeding the web to the precharge Zone.

In experiments conducted with the apparatus of FIG- URE 1, contact roller 43 has been positioned at varying distances from drum 17 and also in light pressure contact with drum 17. The preferred operating conditions'of the apparatus of this invention include spacing contact roller 43 at a slight distance from the surface of the photoconductive insulating layer 18. When operating with the contact roller 43 at the preferred spacing, it has been found that best contrast in images produced is obtained. Further, it has been found that when contact roller 43 is in light pressure contact with drum 17 paper tracking difiiculties increase and since no advantage is noticed when roller 43 contacts drum 17 slight spacing is preferred.

No improvement in contrast or density is noticed when a precharge from precharge mechanism 45 is applied to the surface of web 38 prior to its moving into contact with photoconductive insulating layer 18. However, in some cases when a precharge is not applied portions of the copy were characterized by lines extending across the wveb. These lines disappear when a precharge is applied to web, 38 as it moves over contact roll 43 and, accordingly, precharging during operation of the apparatus of this invention is preferred. Further, it is generally preferred that the polarity of charge applied by precharge mechanism 45 be of a polarity like the polarity of image charge on the surface of drum 17. The application of charge of like polarity is presently believed to prevent uncontrolled charge transfer from the surface of photoeonductive insulating layer 18 to web 38 as web 38 moves into contact with drum 17. i

prises a conductive rubber outside sleeve mounted on an aluminum mandrel and is insulated from other portions of the device illustrated in this figure as, for example, by being mounted and rotating in Teflon bearings. In this figure the roller is indicated as electrically floating. However, various voltages ranging from ground to two thousand volts have been applied to the roller as, for example, from high voltage source 32. For preferred operation, roller 46 is allowed to electrically float as shown with switch 29 open breaking connection to source 32 since occasionally when roller 46 is biased lines not in the original image appear across the images reproduced on web 38. It is to be realized, of course, that these lines may have been due to mechanical conditions in the specific machine tested and may not occur in other models. The success with a floating roller is believed due to the rollers acquisition of charge from the back of web 38 thus creating on roller 46 a potential somewhat higher than the potential on the back of the web for as the web rolls away from drum 17 and follows roller 46 around through a small arc the field will increase between web 38 and roller 46 causing charge transfer between the rear surface of web 38 as it separates from roller 46 until an equilibrium condition is reached with the roller at a raised potential.

Separating roller 46 has been operated while spaced from the drum up to about one-quarter inch and through the various spacings between the one-quarter inch and good pressure contact. It Was noticed that with roller 46 spaced from the drum, images produced were almost as good as with the roller in good pressure contact. How

ever, in many instances background appeared slightly greater with the roller spaced from the drum and, accordingly, the preferred embodiment of this invention in cludes roller as in good pressure contact with drum 17.

As web 38 moves from separating roller 46 there is on its surface a transferred electrostatic charge pattern conforming in configuration to the original electrostatic latent image existing on drum 17 when it moved into transfer station 12. Thus, between the point offeeding to the surfaceof drum 17 around contact roll 43 through the point of separating against separating roller 46 there is transferred from the surface of photoconductive insulating layer 18 to the facing surface of web 38 a charge pattern controlled by the image on the surface of drum 1'7 and controlled also by the electrostatic fields applied to bring about transfer of the electrostatic image.

Following passage beneath precharge electrode 45 web charge wire .14 partially surrounded by shield 9. Corona discharge wire 14 is connected to high voltage source 32 and shield 9 is maintained at ground potential. Preferably, positive polarity charge is sprayed to the back surface of web 33 and this is brought about through the application of several thousand volts positive polarity from voltage source 32 to discharge wire 14.

The rear surface of web 38 is next moved around and in contact with separating roller 46. This roller com- 7 Also, ions may collide with the surfaces themselves which Transfer of images for image formation on web 38 during movement through transfer station 12 is presently believed to be mainly due to a form of gas ionization and/or possibly field emission, in some cases, brought about by the intense electric field in the space between the surfaces of web 38 and photoconductive insulating. layer 18. It is to be realized that although these sur-.

faces have generally been referred to as in physical con tact, they are in reality separated by a minute gap which cannot be avoided when surfaces are in normal physical contact. The electric fields applied between the surfaces,

when sufiiciently intense, accelerate ions existing in the gap into collisions with nearby air molecules or other gaseous molecules thereby creating additional ions which similarly collide with other molecules, create more ions, etc,

define the gap and create additional ions through these collisions, and the various ions travelling in the gap deposit on the surfaces defining the gap as controlled by the electric fields across the gap. Since the fields in areas of image are different than in areas of no image,'there is formed on webfifi a charge pattern conforming in configuration to the charge pattern originally present on photoconductive insulating layer 1% when it enters charge transfer station 12..

The theory of charge transfer which appears to explain how images are formed in the apparatus of this invention is more fully described inouraforementioned pending patent application of which this application is a continuation-impart and the disclosure therein is incorporated into this application by reference. What is pertinent in terms of the present apparatus is that charge transfer for image formation on web 38 may take place while web 38 is in a facing relationship with the surface of photoconductive insulating layer 18 when the field and gap conditions are ripe for controlled breakdown. However, when operating the instant apparatus in the preferred manner, the gap is generally too small for transfer until the surface of web 38 is separated from the surface of photoconductive insulating layer 18. It is believed that transfer of the electrostatic image from layer 13 to web 38 will, in most instances, take place as the web is being peeled away from layer 18 while passing around roller 46 and at a place where the gap is still very small, probably below twenty-five microns. Specific examples of voltage and gap relationship for transfer or breakdown are disclosed in the aforementioned application of which this application is a continuation-in-part and when operating within the zone of charge transfer according to this invention gas ionization is controlled and will in no way destroy the layers defining the gap and is not of a sparking nature.

It is noted that as ionization takes place there is a reduction in the contrast of the original image due to the attraction to the image bearing surface of charge which naturally reduces image contrast. Accordingly, it is generally preferred in operating the present mechanism to initially sensitize the xerographic cylinder 17 to a high level of charge and, following exposure, there will exist a latent image of greater contrast than if the usual level of charge to sensitize were employed and it is believed generally that the more image contrast in the original image the more image contrast in the transferred charge pattern on the surface of web 38 following movement through transfer zone 12.

It is further interesting to note that images have been produced with the apparatus illustrated in FIGURE 1 without charging mechanism 47 operating. Apparently, the field produced through the use of precharge from precharge device 45 as well as the field created between separating roller 46 and the surface carrying the original image is sufficient to produce charge transfer. Generally, however, it is preferred to apply about six to eight thousand volts to discharge electrode 4? while the electrode is positioned in reasonably close proximity to the rear surface of web 33.

Prior to moving the web 38 into the development zone 50 whereat the latent image on the surface of web 38 is visualized through the deposition thereon of developer particles web 38 is moved past neutralizing corona discharge device 48 comprising, as in the case of the other corona devices, of a corona discharge wire partially surrounded by a shield. Neutralizing device 43 has been found particularly valuable and necessary when device 47, the field applying means to bring about image transfenapplies a positive charge to the rear surfaceof web 33, since without neutralization by device 48 substantially no image is obtained and a heavy mottled background is experienced in the images produced. When a negative image transfer field is used no need for the neutralizer 48 appears to exist. To neutralize, A.C. is applied from source 1.3 and A.C. corona is sprayed from discharge device 48 onto the back of web 38 At development zone 50* the transferred electric charge pattern is developed and various known techniques of development may be employed; for example, particles may be cascaded across the surface as isillustrated and described in Walkup U.S. Patent2,618,551 and Schaffert U.S. Patent 2,576,047. Also, the image on the surface of the transfer web may be developed according to techniques illustrated in Carlson US. Patent 2,761,416, or other techdetail in connection with FIGURE 2 and, accordingly, discussion ofits operation will be momentarily postponed. The web having passed through development zone 5t? now carries a developed image of loose particles held in place due to electrostatic fields of force and is next fed over guide roll 51 to position the web for feeding beneath fuser 52 Whereat the particles are fused into a permanent image. If fusible particles are being employed the particles themselves may be fused, and if nonfusible particles are being employed, the web surface may be a material which can be softened, and the web surface may be softened to allow particles to fuse or flow therein or a protective cover may be positioned over the image. When the image is fused to web 38 a heat fuser of the type illustrated in Sabel US. Patent 2,586,484 or a vapor fuser of the type illustrated in Carlson U.S. Patent 2,776,987 may be employed. Also, other techniques to make the image on web 33 permanent may be called upon as, for example, moving the loose particles on the surface of the web between pressure rollers to imbed the particles in the web. After the image is made permanent the web is fed between drive rolls 53 driven by motor 58 through belt an at a linear speed synchronized with the linear speed of drum 17. Optionally, motor 21 may be geared and connected to roller 53 if desired and following passage through rollers 53 web 38 is fed to windup roll 55. It is to be recalled that this invention primarily is concerned with apparatus to transfer charge from one surface to another and, although this embodiment develops, makes permanent, and rolls up the copy obtained, other means of utilizing the image as, for example, scanning or the like are intended to be in cluded within the scope of this invention. Further, rather than passing the web to a takeup roll, it may be cut and stacked or the like.

As pointed out in our aforementioned copending application of which this invention is a continuation-in-part, certain benefits are believed to flow from developing the charge pattern on the web in the form of a loop or employing what has become known in the art of xerography as loop development.

In FIGURE 2 is shown an embodiment of improved loop development mechanism particularly valuable in connection with image development of a transferred charge pattern on a transfer web in accordance with this invention and an improvement over Carlson US. Patent 2,7 61,- 416. As illustrated in this figure, transfer web 38 is fed around discs 61 which are rigidly mounted and spaced apart by an axial hollow shaft 62 and are mounted for free rotation by positioning their axle 63 in bearings of a housing as, for example, is shown in the Carlson aforementioned patent. Positioned within the axial hollow shaft 62 are means to retone and mix developer with new toner and the like as is fully described in the aforementioned Carlson patent and as, for example, may include a pair of concentric hollow shafts, the inner one having a spiral thread inside of it and pickup scoops extending out at one end to pick up toner or a rich toner-carrier mixture and feed it along the spiral to its center where it drops into the outer tube which picks up developer with scoops as, for example, scoop 67 illustrated in FIGURE 2 to pass the developer from scoop 67 to the inside of the larger tube as discs 61 and shaft 62 rotates where the developer mixes with the toner or rich toner-carrier mixture falling from the inner tube to the outer tube, and, after mixing or retoning, the retoned developer is fed from the outer tube out into the channel formed by web 38 around discs 61. The developer particles in the channel tumble and cascade against the facing surface of transfer web 38 and develop, through deposition, the electrostatic charge pattern on this surface. These various elements have all been shown and are thoroughly discussed in the aforementioned Carlson patent.

When development is continuous it has been found that port shafts 66 attached to the frame or housing as, for example, the housing illustrated in FIGURE 1. Positioned againstthe rear surface of development electrode 65 are tilted vanes Development electrode 65, in the device constructed, is a curved metal plate but, as is well-known in the art, may comprise any equipotential surface as, for example, a fine mesh screen, or the like. Electrode 65 is mounted almost concentrically with the loop formed by web 38 but in this embodiment is spaced closer to the web along the electrodes bottom edge than along its top edge.

Employing a development electrode 65, illustrated in FlGURE 2, in a loop development mechanism was found beneficial in a threefold manner. First, it was found that the density of the image was improved. Next, through the use of the vanes 68, tilted as illustrated, means are provided to bring about a substantially uniform, lateral flow of the developer material to the side Where the pickup scoop 67 is located and these results improved uni formity in the developed image. The vanes plus feeding of the new developer into the channel for development purposes at the opposite disc end of the axial hollow shaft 62 provide for uniform movement of the developer from the input feed side to the scoop (67) side of the channel. In operation of the mechanism of FTGURE 2 developer particles are positioned against the surface of sheet 38 as illustrated in FIGURE 1 and as sheet 38 moves around discs 61 the particles tend to move with moving web 38 and thus come between web 38 and electrode 65. They finally fall back along the rear surface of the electrode to a low point in the channel formed by web 38-. The particles are then recycled against web 38 and over the electrode. As the particles continuously cycle they are continuously moved laterally by the tilted vanes 68 provided along the back surface of development electrode es. These vanes 68 provide for continuous and uniform lateral flow of the developer material from the input end to the scoop or pickup end of the channel and this lateral movement improves upon uniformity of development by pro Viding a continuously and substantially uniform amount of developer as well as a substantially uniform mixture of developer for development purposes at all areas along the loop or channel of web 38 being developed. The third major improvement noticed through the use of a development electrode was that without the development electrode it was found that as development continued the carrier particles of the developer mix became excessively charged and the carrier particles adhered to the surface of web 38 as it moved away from the development zone.

These particles tend to distort the image as well as the transfer Web layer. By providing a development electrode and holding it at a grounded or low potential the charge on the carrier is continuously removed by contact with the development electrode and the carrier particles no longer stick to the'surface of Web 38 as the web leaves the development zone.

Although best development has been obtained with the mechanism shown in FIGURE 2, transferred images formed in apparatus as illustrated in FIGURE 1 have been developed according to known development techniques including cascade development, magnetic brush development, and powder cloud development, and it is not intended to limit development in this invention to the particular mechanism shown but it is intended instead to includewithin the scope of this invention all known techniques of image development. 1

Various developer materials d9 were used in the developing zone Sit, and, generally, developers available from The Haloid Company, Rochester, New York, sold under the trademark XeroX work well in this invention. These include, for example, developers disclosed and described in Walkup and Wise US. Patent 2,63 8,416, Walkup-US. Patent 2,618,551, and Wise US. Patent 2,618,552.

In connection with the development zone-and the mechanisrn shown in FIGURE 2 best operation resulted when w the development electrode was spaced inch from the transfer web at the bottom edge of the development else-- trode and 3 inch at the top edge. The electrode itself was about two inches long and had a radius of curvature the same as that of the discs. The width of the electrode: was slightly smaller than the space between the discs to provide clearance between the electrode and the disc as the discs rotate and the electrode remains stationary.

The present invention offers a number of new advantages. When used in connection with xerography there is no need to directly clean untransferred developing powder from the surface of the xerographic drum or plate, even though in the embodiment shown a lint remover brush 57 is included. Cleaning as known today in xerography is cleaning of residual images from the surface of a xerographic plate or drum. This form of cleaning is not necessary in connection with the present invention where development takes place on a diiferent and separated surface. Since no development takes place on the image bearing surface, a xerographic plate would be subject to less abrasion. A xerographic plate in the usual sense is reused through a number of cycles. Naturally, however, the developing material tends to abrade the delicate surface of the xerographic plate and since development in this invention takes place on the transfer web, abrasion of the drum or plate is reduced substantially. There is also present in connection with the present invention the advantage of being able to use new developer materials. Developers used in xerography genorally are somewhat limited by the xerographic photosensitive surface, that is, the developers should not abrade the surface and should not react in any detrimental way with the surface so that the surface may be continuously recycled. Now, however, development is on the surface of a web material intended to be developed only once, and these same considerations do not apply. Also, as compared to xerography as was known prior to this invention, a smaller size machine is now possible. In the past it has been necessary to position additional stations as, for example, the developer station along the line of rotation of the drum, and this necessarily included sufficient area for the developer material to pass in contact with sections of the drum. With development taking place apart from the drum surface and on a different surface, this need for additional space along the drum circumference is removed and allows for the production of smaller and more convenient machines and including machines with smaller drums than has heretofore been possible, thus simplifying handling, reducing cost, and generally improving apparatus in xerography.

As is apparent, a specific embodiment has been described in connection with the present invention and it is to be realized that there is no intention to be limited thereto. Instead, alternatives and equivalents are intended to be included within the scope of this invention. For example, as an alternative to the means of exposure illustrated other known means of exposure may be employed as, for example, the projection of microfilm images or other transparencies, contactv exposure, and the like. Also, although there is illustrated in FIGURE 1 movement of the xerographic drum 17. as Welles-synchronized movement of copy drum 25, these members, when considering the projected image, are in reality stationary relative to one another and it should therefore be immediately apparent that, although a continuously moving apparatus is illustrated, stop-and-go apparatus and the like are also intended to be encompassed within the scope of this invention. Thus and for example, individual frames or individual original copies may be moved into position for exposure according to techniques well-known in the art and, then, while the original copy and the xerographic drum are held stationary, exposure may take place and, following exposure, xero'graphic drum 17 would be moved to the other processing stations accord ing to this invention, and new copy positioned for ex-; posure of thesame copy repeated as desired. Also, it is I to be realized that, although a cylindrically shaped xerographic drum is illustrated in FIGURE 1, there is no intention to be limited thereto. Instead, it is to be realized that drum 17 is for illustrative purposes only and, further, the art of zerography is described in this invention for illustrative purposes only. Other means of image formation may be utilized and, if xerography is used, flat plates or plates of other shapes or forms may be employed without departing from the scope of this invention. Similarly, although the drum being described in connection with this figure is a rigid member, as is Well-known in the art, flexible plate members may also be employed. When other means of forming an electrostatic charge pattern are used such as the selective deposition of charge or the like, the xerographic plate member may be dispensed with and an insulating layer substituted therefor, or, alternatively, a xerographic plate may be employed. When alternative means of charge image formation are embodied in a device, exposure station 23 and sensitizing station 11 of this embodiment are replaced by such other means. Similarly, other variations on other features of this invention are intended to be included within its scope and it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. Apparatus to transfer an undeveloped invisible electrostatic charge pattern from an image bearing surface to a transfer web and to develop and make visible the charge pattern on the transfer web comprising support means for an image surface, means to form an electrostatic latent image on an image surface on said support means, means following latent image formation on the image surface to feed a transfer web into normal physical contact with the image surface on said support means, means to apply an intense electrostatic field between the image surface and the facing surface of said transfer web While said web is in normal physical contact with the image surface, a development zone, and means to separate the transfer web from the image surface and feed the web through said development zone to develop the transferred charge pattern, said development zone including spaced discs pivoted for rotation about a com mon axis so that feeding of the transfer web around a substantial portion of the rims of said discs forms an upwardly concave channel in said transfer web as it passes around said discs, and a development electrode positioned and disposed extending parallel to said axis between said discs and inside and along a portion of the channel formed by said transfer web.

2. Apparatus according to claim 1 in which the development electrode comprises a cylindrical segment and in which means are provided along the near surface of the development electrode to move developer particles axially in the channel formed by said sheet material.

3. Apparatus according to claim 2 in which the development electrode is rigidly positioned with a lower edge closer and with an upper edge farther from the flexible material forming the channel.

4. Apparatus according to claim 3 in which the lower edge of said development electrode is positioned and disposed first in the direction of movement of said sheet material and in which the upper edge is next in the direction of movement of said sheet material.

5. Apparatus according to claim 4 in which the development electrode is positioned and disposed along that portion of movement of the sheet material in which the sheet material is moving in an upward direction.

6. In developing apparatus for applying developing powder to electrostatic latent images on flexible sheet material including a pair of spaced discs pivoted for rotation about a common axis, means to feed a strip of flexible sheet material around and in contact with a substantial portion of the rims of said discs to form an upwardly concave partly cylindrical channel defined by said discs and said sheet material as it passes around said discs, the improvement comprising a development electrode positioned and disposed within said channel extending parallel to said axis between said discs and adjacent a portion of the circumference of the channel formed by said flexible sheet material.

7. In image forming apparatus in which an electrostatic latent image is formed on an insulating web conforming in configuration to an original pre-formed electric image pattern including first and second guide means to place a first surface of the insulating web against the pie-formed electric image pattern and to maintain said web stationary relative to said image pattern, means to apply an intense electric field between said Web and said electric image pattern while said web is maintained relatively stationary against said image pattern, and means to separate said web from said image pattern, the improvement comprising said second guide means being an electrically insulated conductive electrode positioned and disposed relative to said web against said image pattern contacting the second surface of said web as said web is separated from said image pattern, a potential source, and means to connect said second guide means to said potential source.

8. Apparatus according to claim 7 in which said second guide means is electrically floating.

9. Image forming apparatus to form an electrostatic latent image on an insulating web conforming to an original pre-formed electrostatic latent image on an insulating image bearing layer comprising contact means to place a first surface of the insulating web against the surface bearing the pro-formed electrostatic latent image of the image bearing layer, means to maintain said web and said layer stationary relative to each other, an electrostatic field producing means positioned and disposed relative to said web against said image bearing layer adapted to be connected to a potential source to apply an intense electric field between said web and said layer while said web is positioned against said layer, means to separate said web from said layer, and an electrically insulated conductive electrode positioned and disposed relative to said layer to contact the second surface of said web during separation of said web from said layer.

10. Image forming apparatus to form an electrostatic latent image on an insulating Web comprising image forming means to form an electrostatic latent image on an insulating image bearing layer, following image formation on said layer means to move said image bearing layer to and through an image transfer zone, first and second guide means at said image transfer zone to place a first surface of the insulating web against the surface of the image bearing layer bearing the pro-formed electrostatic latent image and to maintain said insulating web and said image bearing layer stationary relative to each other during passage through said image transfer zone, and an electrostatic field producing means positioned and disposed relative to said web against said image bearing layer at said image transfer zone adapted to be connected to an electric potential source to apply an intense electric field between said web and said layer while said web is positioned against said layer, said second guide means being adapted to separate said web from said layer and comprising an electrically insulated conductive electrode positioned and disposed relative to said layer to contact the second surface of said web as said Web is separated from said layer following movement of said layer and said web through said image transfer zone resulting in an electrostatic latent image on said web sep arated from said image bearing layer conforming in configuration to the electrostatic latent image formed on said image bearing layer by said image forming means.

11. Apparatus according to claim 10 in which said image bearing layer includes a photoconductive insulating layer and in which said image forming means includes means to uniformly sensitize and selectively expose said photoconductive insulating layer to an image pat- 11 tern to form an electrostatic latent image on said photoconductive insulating layer conforming to said image pattern.

12. Apparatus according to claim 11 in which said photoconductive insulating layer is selenium and in which said second guide means against which said web is separated from said image bearing layer is electrically biased. 13. Apparatus according to claim 11 in which said photoconductive insulating layer is selenium and in which said second guide means against which said web is separated from said image bearing layer is maintained electrically floating.

l4. Apparatus according to claim 11 in which said second guide means comprises a rotatable cylindrical member positioned and disposed in good pressure contact with the image bearing layer and in which said web moves between said second guide means and said image bearing layer as said web moves with said image bearing layer out of said image transfer zone, said Web then moving with its second surface against said second guide means as said web moves away from said image bearing layer.

15. Apparatus according to claim 11 in which said image forming means includes a corona discharge electrode positioned and disposed relative to said photoconductive insulating layer to apply a uniform charge to the surface of the photoconductive insulating layer to uniformly sensitize said photoconductive insulating layer and in which said electrostatic field producing means at said image transfer zone comprises a corona discharge electrode to apply electrostatic charge to said second surface of said Web at said image transfer zone and in which each discharge electrode applies charge of the same polarity.

16. Image forming apparatus to form an electrostatic latent image on an insulating web conforming to an original preformed electrostatic latent image comprising electrostatic charges of the same polarity on an insulating image bearing layer, said apparatus comprising a precl1arge station including means to electrically charge a first surface of the insulating web to a polarity like the polarity of the electrostatic latent image on the insulating image bearing layer, first and second guide means to place said first surface of the insulating web in a precharged condition against the surface bearing the preformed electrostatic image of the image bearing layer and to maintain said web and said layer stationary relative to each other, and a corona discharge electrode positioned and disposed relative to said web against said image bearing layer adapted to be connected to a potential source to apply an electrostatic charge to the second surface of said web while said web is positioned against said layer, said second guide means being adapted to separate said web from said layer and comprising an electrically insulated and electrically floating conductive electrode positioned and disposed relative to said layer to contact the second surface of said web during separation of said web from said layer.

17. Apparatus according to claim 16 in whichsaid means to charge the first surface of said insulating web comprises a corona. discharge electrode connectedto a potential source facing said first surface and includes a grounded conductive electrode adjacent to said second surface of said web at said prc-charged station.

References Cited in the file of this patent UNITED STATES PATENTS 2,297,691 Carlson Oct. 6, 1942 2,576,047 Schaifert Nov. 20, 1951 2,726,940 Buhler Dec. 13, 1955 2,752,833 Jacob July 3, 1956 2,761,416 Carlson Sept. 4, 1956 2,777,745 McNaney Jan. 15, 1957 2,824,813 Fauser et al. Feb. 25, 1958 2,825,814 Walkup Mar. 4, 1958 2,857,290 Bolton ct. 21, 1958 2,913,353 Mayer et al. Nov. 17, 1959 FOREIGN PATENTS 734,909 Great Britain Aug. 10, 1955 1,112,180 France Mar. 9, 1956 

